Update to JDK 21 (Maven compiler plugin)

What's Changed

• New .gitignore by @bkiscool in #240
  Removed a bunch of IDE files that shouldn't be tracked
• Update to JDK 21 by @bkiscool in #241
  Maven compiler plugin is now version 21

Full Changelog: v9.0.3...v9.0.10

Made for Minecraft version 1.21.8
Made for PGF Season 13